The present invention relates to contactors, and more particularly to improved means for controlling the operation of the arc-quenching circuit of a contactor.
Contactors are a type of electrical switching device which is adapted to make and break relatively high currents over and over again, often sustaining arcing, heat and wear. Unlike circuit breakers, which are also adapted to break high currents, contactors ordinarily need only be able to interrupt normal-level load current. Unlike circuit breakers, however, a contactor must be able to operate repeatedly in normal service without periodic inspection or replacement of parts. Further, unlike a circuit breaker, a contactor must be able to close an energized circuit, sustaining full load current each time. Since the device is always in the circuit it must present as small an impedance as possible in order to avoid losses and undesirable heating.
Normally contactors are provided with arc chutes and associated electromagnetic field for urging an arc upward into the chute, where it is stretched, cooled and extinguished. Magnetomotive force is provided by a coil, termed a "blowout coil", which is ordinarily in series with an auxiliary contact. When the contactor opens, the auxiliary contact initially stays closed and carries load current so that the main contacts may separate with little or no arcing. The auxiliary contacts then open, drawing an arc between them. The current flowing through the auxiliary contacts, however, is also flowing through the blowout coil and thus produces a magnetic field which urges the arc away from the auxiliary contacts and eventually extinguishes it. When the main contacts again close, the auxiliary contacts also close so that the blowout coil is connected in parallel about the main contacts. Due to the resistance inherent in the coil (and to some extent in the auxiliary contacts) practically all of the load current flows through the main contacts.
After many operations, however, the main contacts may become contaminated or dirty due to the environment in which the contactor is used. This dirt and contamination then raises the effective resistance of the main contacts to the point where significant amounts of load current begin to be diverted through the blowout coil circuit. The result is to give rise to I.sup.2 R heating in the coil, ultimately causing it to fail because of destroyed insulation or even melted conductor material. Accordingly, it will be appreciated that it would be highly desirable to provide a control system for a contactor which precludes the shunting of load current through a blowout coil due to heightened resistance of main contacts.
It is therefore an object of the invention to provide an improved contact sequencing system.
Another object is to effect the disconnection of a blowout coil during closure of a set of contactor main contacts.
Yet another object is to provide a contactor with a simple, reliable mechanism for operating multiple contacts in the necessary sequence for disconnecting a blowout coil when its presence is unnecessary to the operation of the system.